Microbiome of Saharan dust aerosols

Airborne biological material, or bioaerosol, plays an important role in the Earth system and therefore have an impact on the atmosphere, the biosphere and the hydrological cycle as well as on public health. Bioaerosols consist of viruses, bacteria, mold, pollen, plant fibers and fragments that range from tens of nanometers to a few hundred micrometers in size. Terrestrial ecosystems are the major sources of the atmospheric bioaerosols with urban environments and areas with agricultural and industrial activity being particularly important. Desert dust contains high concentrations of bioaerosol mainly composed of soil microorganisms and plant detritus. This dust may be further enriched with fungal spores, bacteria, viruses, and pollen that accumulate as dust plumes are transported over terrestrial and aquatic environment through the adhesion of microbe-laden fine aquatic sprays to dust particles. The relative importance of bioaerosol sources in the atmosphere varies with altitude, season, location and meteorological factors.

In this study, Saharan dust aerosols (n=19) were sampled from East Mediterranean (Crete, Greece) using a high-volume TSPs sampler (TISCH). Dust atmospheric particles were collected on precombusted (450 °C for 5 h) 20 × 25 cm quartz filters (Pall, 2500QAT-UP). The sampling resolution was 48 h, at a flow rate of 85 m³ h⁻¹. We established a reliable analytical protocol for extracting DNA from these Saharan dust particles. Together with biological quantification and identification, chemical analysis was performed, including metals, major ions, phospholipids and sugars.

Results show that the number of Eucaryotic DNA copies were 30 times higher than the bacterial copies during the dust events. The bacterial community composition in the collected dust aerosol as the most abundant Phyla were Proteobacteria (37%) followed by Actinobacteria (22%) and Firmicutes (13%). Furthermore, we analyzed five (n=5) intense dust events using magic angle spinning solid-state ³¹P-NMR. The results show that the typical functional groups in P speciation, were orthophosphate and monophosphate esters which sharing the same chemical shift (H₃PO₄ and RH₂-PO₄), phosphate diesters (R₁R₂ HPO₄) and pyrophosphate (H₄P₂O₇). No phosphonates were detected (C-P bond) in dust samples. Monophosphate esters and diesters are mainly found in nucleotides and their derivatives (e.g., DNA, RNA, AMP, ADP and ATP) but also in phospholipids, and as such, they constitute the majority of atmospheric organic-P. These organic-P compounds have C-O-P bonds and are easily hydrolyzed in the marine environment by alkaline phosphatase enzyme, providing an important source of P in the aquatic ecosystems when Saharan dust is deposited.